mfvideoenc: Don't pass 0/1 framerate to MFT

Some MFT implementations do not accept 0/1 framerate and it will
result in encoder open failure. If framerate is unknown,
we will use arbitrary 25/1 framerate value.

Part-of: <gstreamer/gst-plugins-bad!2109>
29 jobs for !2109 with backport-mr-2106-into-1.18-mft-framerate in 27 minutes and 41 seconds (queued for 3 seconds)
merge request